1. Field of the Invention
The present invention pertains to methods for the formation of channels for sonars, notably for so-called relocalization sonars which enable precise localization of an object that has previously been roughly localized by a sonar.
If the relocalization sonar is then placed in a consumable piece of equipment, i.e. in one that is either destroyed or abandoned after use, the sonar is itself destroyed or abandoned. It is therefore necessary, in this case, to minimize its cost. This is made possible, in particular, by accepting characteristics inferior to those of localization and identification sonars since substantial information is already available on the localization and characteristics of the targets.
Typically, a relocalization sonar works at a frequency of the order of some hundreds of kHz, for example 500 kHZ, with a range of some tens of meters, for example between 50 and 100 m.
The performance characteristics sought then are, for example, an observation sector of about 60.degree., an angular resolution of between 1.degree. and 2.degree. and a resolution in distance of about 10 cm. Furthermore, the image rate should remain relatively high, 1 image/s at the minimum.
2. Description of the Prior Art
The various known approaches do not allow for meeting these conditions satisfactorily.
Thus, there is a known way of forming reception channels electronically, but this necessitates omnidirectional transmission throughout the sector of observation, and this requires high electrical power. Furthermore, a large number of channels (several tens of channels) have to be formed to obtain the desired resolution. This necessitates a proliferation of equipment and the use of a delay technique, and not a phase-shifting technique, owing to the high values of the offsets to be obtained. This approach is therefore very costly.
There is also a known way of making a directional transmission antenna that meets the conditions of the requisite angular resolution and, therefore, uses up little electrical energy because of the antenna gain. This antenna rotates with the reception antenna, but it is then no longer possible to obtain the desired image rate since it is necessary to wait for the return of the pulse before making the antenna rotate by the increment corresponding to the resolution.
Finally, there is a known way of making a sonar, described in the French patent No. 2 553 395, invented by Georges GRALL, filed on 25th Oct. 1983 by the present Applicant. This sonar has a stack of angularly offset antennas, each of which determines a sub-sector. This enables the antenna to be rotated at a sufficient speed to obtain the desired image rate. However, the volume of the equipment in such a system is great, and can be reduced to a reasonable value only for frequencies higher than those of the MHz range.